The extent to which the quasi-biennial oscillation (QBO) of the equatorial stratosphere affects tropical systems has been an open question for some time, but little has been resolved. In recent years, there has been great interest in dynamical interactions between the stratosphere and the troposphere, but little work on how stratospheric changes affect tropical deep convection. Some forecasts of Atlantic hurricane activity have used the QBO as one of the predictors, but the physical connection between the QBO and hurricanes has not been explored. This project is an effort to make systematic advances on these problems.
Analyses of observations and modeling will be used to investigate the possible influence of the QBO on tropical deep convection. Previous observational analyses and general circulation studies found significant effects of the QBO on tropical deep convection; this project addresses several weaknesses in these studies. A cloud product that eliminates the ambiguity between deep convection and cirrus clouds in OLR (Outgoing Longwave Radiation) data will be used, and ENSO (El Nino-Southern Oscillation) influences on deep convection will be separated from those of the QBO. Because it is difficult, if not impossible, to address causality through statistical studies, cloud-resolving and single-column modeling will be employed to investigate how the QBO might affect tropical deep convection. Background states for the cloud resolving model will be obtained from high vertical-resolution radiosonde data.
Broader impacts of this project include the training of a PhD student in a project that combines multiple research approaches: statistical analysis, cloud-resolving modeling, and general circulation modeling, and using state-of-the-art high-performance computing. This work should shed light on the physical basis for one of the predictors of Atlantic hurricane activity and on the vertical resolution needed in models of the upper troposphere-lower stratosphere region to properly account for upper troposphere-lower stratosphere interactions with deep tropical convection.
The quasi-biennial oscillation (QBO) is a phenomenon that describes the wind reversal in the equatorial lower stratosphere from eastward to westward with a period that has averaged about 27 months since the QBO data record began in 1953. The QBO occurs primarily in the stratosphere and has its maximum amplitude at about 25-30 km altitude, but its influence extends downward into the upper troposphere. This is clear, given the known QBO modulation of tropical tropopause (the boundary between the troposphere below and the stratosphere above) temperatures. Our paper Liess and Geller (2012) expanded on previous work to show that there was a QBO modulation of tropical clouds and precipitation in regions where tropical precipitation and cloud heights were the highest. This has some potential societal importance in that Gray (1984) has suggested that there is a QBO modulation in Atlantic hurricanes, and it has been verified by Camargo and Sobel (2010) that this relation did exist before the 1980s, but not afterward. In addition, Ho et al. (2009) demonstrated a dependence of western North Pacific typhoon tracks on the phase of the QBO. Another important aspect of the QBO is its modulation of tropical cold-point tropopause (CPT) temperatures, since these are related to the moisture of air entering the stratosphere. The amount of stratospheric water vapor has been shown to be important for both stratospheric ozone chemistry and for global warming. Since Taguchi (2010) demonstrated that both the period and amplitude of the QBO depended on the phase of ENSO (El Niño/Southern Oscillation), our paper by Yuan et al. (2013) has shown that the QBO modulation of tropical CPT temperatures are substantially larger during the La Niña phase of ENSO than during its El Niño phase. This has important implications for stratospheric water vapor. Finally, we have performed cloud-resolving model numerical experiments that have demonstrated that observed QBO structures can alter the distribution of high clouds in the tropics. These results have yet to be published. References Camargo, S. J. and A. H. H. Sobel, 2010: Revisiting the Influence of the Quasi-Biennial Oscillation on Tropical Cyclone Activity. J. Climate, 23, 5810–5825. Gray, W. M., 1984: Atlantic Seasonal Hurricane Frequency. Part I: El Niño and 30 mb Quasi-Biennial Oscillation Influences. Mon. Wea. Rev., 112, 1649–1668. Ho, C.-H., H.-S. Kim, and S.-W. Son, 2009: Influence of stratospheric quasi-biennial oscillation on tropical cyclone tracks in the western North Pacific. Geophys. Res. Lett., 36, doi: 10.1029/2009GL037163 Liess, S. and M. A. Geller, 2012: On the relationship between QBO and distribution of tropical deep convection. J. Geophys. Res., 117, D3, doi:10.1029/2011JD01631 Taguchi, M., 2010, Observed connection of the stratospheric quasi-biennial oscillation with El Niño–Southern Oscillation in radiosonde data. J. Geophys. Res., 115, D18120, doi:10.1029/2010JD014325. Yuan, W., M. A. Geller, P. T. Love, 2013: ENSO Influence on QBO Modulations of the Tropical Tropopause. Q. J. Roy. Meteorol. Soc., doi: 10.1002/qj.2247.
